it could be braxton hicks. labour contraction will be as similar as what you described, but the cramp feeling will continue to intensify, and it's consistent also it happens regardless you're moving or not. but if you're unsure, you can always visit your doc to reconfirm.
Back pain closer to EDD is normal. However sciatica usually will be left side, left leg and pain will be at your butt. I had sciatica for second pregnancy from second tri till birth.